Laminated dielectric filter

Abstract: A laminated dielectric filter is formed by strip line resonators on a plurality of dielectric sheets and electric field or magnetic field coupling windows on intermediate dielectric sheets.

Description: This invention relates to a laminated dielectric filter used mainly in antenna duplexers of high frequency radio devices such as mobile telephones. An antenna duplexer is a device for sharing one antenna by a transmitter and a receiver, and it is composed of a transmission filter and a reception filter. The invention is particularly directed to a laminated dielectric filter having a laminate structure by laminating a dielectric sheet and an electrode layer and baking into one body. Along with the advancement of mobile communications, recently, the antenna duplexer is used widely in many hand-held telephones and car-mounted telephones. An example of a conventional antenna duplexer is described below with reference to a drawing. Fig. 3 is a perspective exploded view of a conventional antenna duplexer. In Fig. 3, reference numerals 701 to 706 are dielectric coaxial resonators, 707 is a coupling substrate, 708 is a metallic case, 709 is a metallic cover, 710 to 712 are series capacitors, 713 and 714 are inductors, 715 to 718 are coupling capacitors, 721 to 726 are coupling pins, 731 is a transmission terminal, 732 is an antenna terminal, 733 is a reception terminal, and 741 to 747 are electrode patterns formed on the coupling substrate 707. The dielectric coaxial resonators 701, 702, 703, series capacitors 710, 711, 712, and inductors 713, 714 are combined to form a transmission band elimination filter. The dielectric coaxial resonators 704, 705, 706, and coupling capacitors 715, 716, 717, 718 compose a reception band pass filter. One end of the transmission filter is connected to a transmission terminal which is electrically connected with a transmitter, and the other end of the transmission filter is connected to one end of a reception filter, and is also connected to an antenna terminal electrically connected to the antenna. The other end of the reception filter is connected to a reception terminal which is electrically connected to a receiver. The operation of an antenna duplexer is described below. First of all, the transmission band elimination filter shows a small insertion loss to the transmission signal in the transmission frequency band, and can transmit the transmission signal from the transmission terminal to the antenna terminal while hardly attenuating it. By contrast, it shows a larger insertion loss to the reception signal in the reception frequency band, and reflects almost all input signal in the reception frequency band, and therefore the reception signal entering from the antenna terminal returns to the reception band pass filter. On the other hand, the reception band filter shows a small insertion loss to the reception signal in the reception frequency band, and transmits the reception signal from the antenna terminal to the reception terminal while hardly attenuating it. The transmission signal in the transmission frequency band shows a large insertion loss, and reflects almost all input signal in the transmission frequency band, so that the transmission signals coming from the transmission filter is sent out to the antenna terminal. In this design, however, in manufacturing dielectric coaxial resonators, there is a limitation in fine processing of ceramics, and hence it is hard to reduce its size. Downsizing is also difficult because many parts are used such as capacitors and inductors, and another problem is the difficulty in lowering the assembling cost. The dielectric filter is a constituent element of the antenna duplexer, and is also used widely as an independent filter in mobile telephones and radio devices, and there is a demand that they be smaller in size and higher in performance. Referring now to a different drawing, an example of a conventional block type dielectric filter possessing a different constitution from the above described structure is described below. Fig. 4 is a perspective oblique view of a block type dielectric filter of the prior art. In Fig. 4, reference numeral 1200 is a dielectric block, 1201 to 1204 are penetration holes, and 1211 to 1214, and 1221, 1222, 1230 are electrodes. The dielectric block 1200 is entirely covered with electrodes, including the surface of the penetration holes 1201 to 1204, except for peripheral parts of the electrodes on the surface of which the electrodes 1221, 1222 and others are formed. The operation of the thus constituted dielectric filter is described below. The surface electrodes in the penetration holes 1201 to 1204 serve as the resonator, and the electrode 1230 serves as the shield electrode. The electrodes 1211 to 1214 are to lower the resonance frequency of the resonator composed of the electrodes in the penetration holes, and functions as the loading capacity electrode. By nature, a 1/4 wavelength front end short-circuit transmission line is not coupled at the resonance frequency and shows a band stop characteristic, but by thus lowering the resonance frequency, an electromagnetic field coupling between transmission lines occurs in the filter passing band, so that a band pass filter is created. The electrodes 1221, 1222 are input and output coupling capacity electrodes, and input and output coupling is effected by the capacity between these electrodes and the resonator, and the loading capacity electrode. The operating principle of this filter is a modified version of a comb-line filter disclosed in the literature (for example, G.L. Matthaei, "Comb-Line Band-pass Filters of Narrow or Moderate Bandwidth"; the Microwave Journal, August 1963). The block type filter in this design is a comb-line filter composed of a dielectric ceramic (for example, see U. S. Patent 4,431,977). The comb-line filter always requires a loading capacity for lowering the resonance frequency in order to realize the band pass characteristic. Fig. 5 shows the transmission characteristic of the comb-line type dielectric filter in the prior art. The transmission characteristic shows the Chebyshev characteristic increasing steadily as the attenuation outside the bandwidth departs from the center frequency. In this construction, however, it is not possible to realize the elliptical function characteristic possessing the attenuation pole near the bandwidth of the transmission characteristic, and hence the range of selection is not sufficient for filter performance. Also, in such dielectric filter, for smaller and thinner constitution, the flat type laminate dielectric filter that can be made thinner than the coaxial type is expected henceforth, and several attempts have been made to design such a device. A conventional example of a laminated dielectric filter is described below. The following explanation relates to a laminated "LC filter" (trade mark) that is put into practical use as a laminated dielectric filter by forming lumped element type capacitors and inductors in a laminate structure. Fig. 6 is a perspective exploded view showing the structure of a conventional laminate "LC filter". In Fig. 6, reference numerals 1 and 2 are thick dielectric layers. On a dielectric sheet 3 are formed inductor electrodes 3a, 3b, and capacitor electrodes 4a, 4b are formed on a dielectric sheet 4, capacitor electrodes 5a, 5b on a dielectric sheet 5, and shield electrodes 7a, 7b on a dielectric sheet 7. By stacking up all these dielectric layers and dielectric sheets together with a dielectric sheet 6 for protecting the electrodes, an entirely laminated structure is formed. The operation of the thus constituted dielectric filter is described below. First, the confronting capacitor electrodes 4a and 5a, and 4b and 5b respectively compose parallel plate capacitors. Each parallel plate capacitor functions as a resonance circuit as connected in series to the inductor electrodes 3a, 3b through side electrodes 8a, 8b. Two inductors are coupled magnetically. The side electrode 8b is a grounding electrode, and the side electrode 8c is connected to terminals 3c, 3d connected to the inductor electrode to compose a band pass filter as input and output terminals (for example, Japanese Laid-open Patent No. 3-72706(1991)). In such a constitution, however, when the inductor electrodes are brought closer to each other to narrow the interval in order to reduce in its size, the magnetic field coupling between the resonators becomes too large, and it is hard to realize a favorable band pass characteristic narrow in the bandwidth. It is moreover difficult to heighten the unloaded Q value of the inductor electrodes, and hence the filter insertion loss is large. Another different conventional example of a laminated dielectric filter is described below with reference to an accompanying drawing. Fig. 7(a) and (b) shows the structure of a conventional laminated dielectric filter. In Fig. 7(a) and (b), 1/4 wavelength strip lines 820, 821 are formed on a dielectric substrate 819. Input and output electrodes 823, 824 are formed on the same plane as the strip lines 820, 821. The strip line 820 is composed of a first portion 820a (L 1 indicates the length of 820a) having a first line width W 1 (Z 1 indicates the characteristic impedance of W 1 ) confronting the input and output electrodes 823, a second portion 820b (L 2 indicates the length of 820b) having a second line width narrower than the first line width W 1 , and a third portion 820c having a third line width narrower than the first line width W 1 but broader than the second line width W 2 (Z 2 indicates the characteristic impedance of W 2 ). Similarly, the strip line 821 is composed of a first portion 821a having a first line width W 1 confronting the input and output electrodes 824, a second portion 821b having a second line width narrower than the first line width W 1 , and a third portion 821c having a third line width narrower than the first line width W 1 but broader than the second line width W 2 . The strip lines 820, 821 are connected with a short-circuit electrode 822, and the resonator 801b is in a pi-shape. A dielectric substrate 819 is covered by grounding electrodes 825, 826 at both surfaces. At one side 819a, side electrodes 827,828 are formed, and the grounding electrodes 825, 826, and short-circuit electrodes 822 are connected. On the other side 819b, side electrodes to be connected with the input and output electrodes 823, 824 respectively are formed. The strip lines 820, 821 are capacitively coupled with the input and output electrodes 823, 824, respectively, thereby constituting a filter as described for example, in U. S. Patent 5,248,949. In such constitution, however, same as the conventional block type dielectric filter, the elliptical function characteristic possessing the attenuation pole near the passing band of the transmission characteristic cannot be realized, and hence the scope of performance of the filter is not wide enough. From "Multi-Layered Planar Filters Based on Aperture Coupled, Dual Mode Microstrip or Stripline Resonators", 1992, IEEE, International Microwave Symposium Digest, pages 1203 to 1206 stacked planar filters are known. The stacked planar filters described in this paper can be based on a variety of dual mode, planar resonator structures similar to those used in dual mode microstrip filters. These include square patches, circular disks, and rings. Coupling between the dual orthogonal modes supported by these resonators is accomplished by introducing a perturbation to the symmetry of the previously single mode resonator at a location that is offset 45 degrees from the axes of coupling to and from the resonator. Some possible perturbations can be used to control the coupling between the orthogonal modes supported by a resonator. In the novel filter configurations introduced in this paper, the dual mode stripline resonators are stacked. Coupling energy between the resonators is implemented by including a coupling aperture or iris in the ground plane shared by the two resonators. Both square and circular dual mode resonators are coupled together by either round coupling holes or orthogonal slots. A four pole filter is realised by stacking four patterned substrates directly on top of each other. This concept can obviously be extended to realise filters of any number of poles. EP-A1-0 499 643 relates to a band-pass filter. A triplet line is constituted of a resonance element formed by interposing a dielectric member between a pair of ground conductors, the length of the line is selected to be about 1/4 of the wavelength, and resonators with one end grounded are combined to constitute a band-pass filter. Each resonator is isolated by a separator to prevent a waveguide mode in the triplet line. A plurality of triplet lines are superposed, and the electromagnetic coupling among the resonators is accomplished by a coupling means provided in the ground conductor and the dielectric member. Resonators at both terminals are coupled to input and output terminals. It is a primary object of the invention to provide a laminated dielectric filter at low cost which has an excellent band pass characteristic with small insection loss and high bandwidth selectivity. Another object is to provide a laminated dielectric filter having a small and thin flat structure. The invention provides a laminated dielectric filter as specified in claim 1. In the laminated dielectric filter of the embodiment, it is easy to control from a large coupling degree to a small coupling degree, the size, shape and position of the coupling window, so that a filter characteristic in a wide range from wide band to narrow band can be attained easily. Fig. 1 is a perspective exploded view of a laminated dielectric filter in a embodiment of the invention. Fig. 2 (a) is a sectional view of section A-A' of the laminated dielectric filter in the embodiment of the invention in Fig. 1, and Fig. 2(b) is a sectional view of section B-B'. Fig. 3 is a perspective exploded view of a dielectric antenna duplexer of the prior art. Fig. 4 is a perspective view of a block dielectric filter of the prior art. Fig. 5 is a graph showing transmission characteristic and reflection characteristic of a comb-line dielectric filter of the prior art. Fig. 6 is a perspective exploded view of a laminated LC filter of the prior art. Fig. 7 (a) and (b) is a perspective view of a laminated dielectric filter of the prior art. Example A laminated dielectric filter in an embodiment of the invention is described below by referring to the accompanying drawings. Fig. 1 is a perspective exploded view of the laminated dielectric filter in the embodiment of the invention. Fig. 2 (a) is a sectional view of section A-A' in Fig. 1, and Fig. 2 (b) is a sectional view of section B-B' In Fig. 1, reference numerals 350a, 350b, 350c, 350d, 350e, 350f, 350g, 350h, 350i, 350j indicate dielectric sheets. Reference numerals 351a, 351b, 351c are strip line resonator electrodes. 353a, 353b are input and output coupling capacity electrodes, 354a, 354b are shield electrodes, and 355a, 355b are coupling shield electrodes, which are formed of inner electrodes laminated on the dielectric sheets. Side electrodes 357a, 357b as input and output terminals, and side electrodes 358a, 358b, 358c, 358d as grounding terminals are formed of outer electrodes baked after application of metal paste. The shield electrodes are connected and grounded to the side electrodes 358a. 358b of the side grounding terminals and side electrode 385c of grounding terminal of open end side, aside from the side electrode 358d of grounding terminal at grounding end side. The grounding ends of strip line resonator electrodes 351a, 351b, 351c are connected and grounded to the side electrode 358d of the grounding terminal at the grounding end side through grounding electrodes 352a, 352b, 352c. A parallel flat plate capacitor composed between the input and output coupling capacity electrode 353a and strip line resonator electrode 351a, and a parallel flat plate capacitor composed between the input and output coupling capacitor composed between the input and output coupling capacity electrode 353b and strip line resonator electrode 351c both function as input and output coupling capacitors. The input and output coupling capacity electrodes 353a, 353b are connected to input and output terminals 357a, 357b formed of side electrodes. In the embodiment the coupling amount between the strip line resonators is controlled the electric field coupling windows or the magnetic field coupling windows 356a, 356b formed in the coupling shield electrodes 355a, 355b. Depending on the size, shape and position of the coupling window, it is easy to control from a large coupling amount to a small coupling amount, so that a filter characteristic in a broad range from wide band to narrow band is realized. By capacity coupling for input and output coupling, the design is easy, and the filter size can be reduced. Thus, according to the embodiment a filter characteristic in a broad range from wide band to narrow band can be attained by a simple design.

Claims: A laminated dielectric filter formed by front end short-circuit strip line resonator electrodes (351a, 351b, 351c) on a plurality of first dielectric sheets (350c, 350e, 350g) comprising coupling shield electrodes (355a, 355b) possessing electric field coupling windows or magnetic field coupling windows (356a, 356b, respectively) on a different plurality of second dielectric sheets (350d, 350f), wherein the first dielectric sheets and second dielectric sheets are alternately laminated by aligning the direction of short-circuit ends of the strip line resonator electrodes and the coupling shield electrodes (355a, 355b) are grounded, and comprising shield electrodes (354a, 354b) on second dielectric sheets (350a, 350i) laminated above and beneath the filter, wherein the resonator electrodes are quarter wavelength one end short-circuit type resonators, characterised in that the coupling windows (356a, 356b) are located close to a short-circuit end side of the resonator electrodes, are oblong in the direction perpendicular to the resonator electrodes and are for performing magnetic field coupling.

Inventors: AIZAWA KIMIO, FUJINO TAKASHI, ISHIZAKI TOSHIO, KUSHITANI HIROSHI, NAKAKUBO HIDEAKI, NAKAMURA TOSHIAKI, SASAKI ATSUSHI, SATOH YUKI, AIZAWA, KIMIO, FUJINO, TAKASHI, ISHIZAKI, TOSHIO, KUSHITANI, HIROSHI, NAKAKUBO, HIDEAKI, NAKAMURA, TOSHIAKI, SASAKI, ATSUSHI, SATOH, YUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: H01P1
IPC codes: H01P1